Method and apparatus for completely hiding refresh operations in a dram device using clock division

ABSTRACT

A method and apparatus for handling the refresh of a DRAM array or other memory array requiring periodic refresh operations so that the refresh does not require explicit control signaling nor handshake communication between the memory array and an external accessing client. The method and apparatus handles external accesses and refresh operations such that the refresh operations do not interfere with the external accesses under any conditions. As a result, an SRAM compatible device can be built from DRAM or 1-Transistor cells. A clock division scheme is implemented to perform external accesses during one portion of a clock cycle, and required refresh operations during another portion of the same clock cycle.

RELATED APPLICATIONS

[0001] The present application is a continuation-in-part of U.S. patent application Ser. No. 09/405,607, by Wingyu Leung, entitled “Read/Write Buffers for Complete Hiding of the Refresh of a Semiconductor Memory and Method of Operating Same” filed Sep. 24, 1999, which is a continuation-in-part of U.S. Pat. No. 5,999,474, by Wingyu Leung and Fu-Chieh Hsu, entitled “Method and Apparatus for Complete Hiding of the Refresh of a Semiconductor Memory” issued Dec. 7, 1999.

[0002] The present application is further related to U.S. Pat. No. 6,028,804, by Wingyu Leung, entitled “Method and Apparatus for 1-T SRAM Compatible Memory” and issued Feb. 22, 2000; U.S. Pat. No. 6,222,705, by Wingyu Leung, entitled “Method and Apparatus For Refreshing A Semiconductor Memory using Idle Memory Cycles” issued Apr. 24, 2001; and U.S. Pat. No. 6,075,740, by Wingyu Leung, entitled “Method and Apparatus for Increasing The Time Available for Refresh For 1-T SRAM Compatible Devices”, issued Jun. 13, 2000. These patents are hereby incorporated by reference.

FIELD OF THE INVENTION

[0003] The present invention is related to semiconductor memories, especially dynamic random access memory (DRAM). In particular, the present invention relates to a method and apparatus of handling refresh operations in a semiconductor memory such that the refresh operations do not interfere with external access operations.

DISCUSSION OF RELATED ART

[0004] A conventional DRAM memory cell, which consists of one transistor and one capacitor, is significantly smaller than a conventional SRAM cell, which consists of 4 to 6 transistors. However, data stored in a DRAM cell must be periodically refreshed, while the data stored in an SRAM cell has no such requirement. Each refresh operation of a DRAM cell consumes memory bandwidth. It is possible for an external access to be initiated at the same time that a refresh access is required. In this case, the external access must be delayed until after the refresh access has been performed. In general, this prevents DRAM cells from being operated as SRAM cells, because SRAM cells do not require refresh operations.

[0005] Previous attempts to use DRAM cells in SRAM applications have been of limited success for various reasons. For example, one such DRAM device has required an external signal to control refresh operations. (See, 131,072-Word by 8-Bit CMOS Pseudo Static RAM, Toshiba Integrated Circuit Technical Data (1996).) Moreover, external accesses to this DRAM device are delayed during the memory refresh operations. As a result, the refresh operations are not transparent and the resulting DRAM device cannot be fully compatible with an SRAM device.

[0006] In another prior art scheme, a high-speed SRAM cache is used with a relatively slow DRAM array to speed up the average access time of the memory device. (See, U.S. Pat. No. 5,559,750 by Katsumi Dosaka et al, and “Data Sheet of 16 Mbit Enhanced SDRAM Family 4M×4, 2M×8, 1M×16” by Enhanced Memory Systems Inc., 1997.) The actual access time of the device varies depending on the cache hit rate. Circuitry is provided to refresh the DRAM cells. However, the refresh operation is not transparent to external accesses. That is, the refresh operations affect the memory access time. Consequently, the device cannot meet the requirement of total deterministic random access time.

[0007] Other prior art schemes use multi-banking to reduce the average access time of a DRAM device. Examples of multi-banking schemes are described in “Data sheet, MD904 To MD920, Multi-bank DRAM (MDRAM) 128K×32 to 656K×32” by MoSys Inc., 1996, and in “An Access-Sequence Control Scheme to Enhance Random-Access Performance of Embedded DRAM's” by Kazushige Ayukawa et al, IEEE JSSC, vol. 33, No. 5, May 1998, pp. 800-806. These multi-banking schemes do not allow an individual memory bank to delay a refresh cycle. Another prior art scheme uses a read buffer and a write buffer to take advantage of the sequential or burst nature of an external access. An example of such a prior art scheme is described in U.S. Pat. No. 5,659,515, entitled “Semiconductor Memory Device Capable of Refresh Operation in Burst Mode” by R. Matsuo and T. Wada. In this scheme, a burst access allows a register to handle the sequential accesses of a transaction while the memory array is being refreshed. However, this scheme does not allow consecutive random accesses. For example, the memory cannot handle a random access per clock cycle.

[0008] Another prior art scheme that attempts to completely hide refresh operations in a DRAM cell includes the scheme described in U.S. Pat. No. 5,642,320, entitled “Self-Refreshable Dual Port Dynamic CAM Cell and Dynamic CAM Cell Array Refreshing Circuit”, by H. S. Jang. In this scheme, a second port is added to each of the dynamic memory cells so that refresh can be performed at one port while a normal access is carried out at the other port. The added port essentially doubles the access bandwidth of the memory cell, but at the expense of additional silicon area.

[0009] Another prior art scheme that attempts to completely hide the refresh operations in an asynchronous DRAM is described in U.S. Pat. No. 4,549,284, entitled “Dynamic MOS Random Access Memory”, by Kunihiko Ikuzaki. In this scheme, an automatic refresh circuit is incorporated in an asynchronous DRAM to generate a refresh cycle after an external access cycle is performed. In the absence of an external access, an internal oscillator continues to generate refresh cycles. Thus, the memory device is constantly performing refresh operations, thereby wasting power.

[0010] Moreover, the oscillation period of the oscillator is set by the transconductance of an MOS transistor and a capacitor, which varies with process and temperature. Within a typical process and commercial temperature range, the oscillation period varies by up to a factor of two. As a result, it becomes difficult to synchronize the external accesses and the refresh operations. For this reason, the memory device is not suitable for high-speed operations in the auto-refresh mode.

[0011] Accordingly, it would be desirable to have a DRAM device that handles refresh operations in a manner that is completely transparent to an external accessing memory client for both low-speed and high-speed operations. It would further be desirable if such a DRAM device only performed refresh operations at the times when the memory cells need to be refreshed (i.e., at a rate determined mainly by the charge leakage mechanism of the memory cells, and not by the circuit operation of the automatic refresh circuit).

SUMMARY

[0012] Accordingly, the present embodiment provides a memory device (or an embedded memory block) that includes a plurality of memory cells, which must be periodically refreshed in order to retain data values. In one embodiment, the memory cells are DRAM cells arranged in an array having a plurality rows and columns. In a particular embodiment, the array is divided into a plurality of banks.

[0013] The memory device includes a plurality of terminals for receiving signals from an external accessing client. These signals can include a clock signal, an address signal, a write/read indicator signal, and address strobe signal and a reset signal. However, these signals do not include a signal that indicates that a refresh operation must be performed.

[0014] To implement refresh operations, the memory device includes a refresh controller that periodically asserts a refresh request signal, which is used to indicate that a refresh operation is pending. The refresh controller also provides a refresh address identifying one of the rows of the array. The refresh controller increments the refresh address each time that the refresh request signal is asserted.

[0015] The memory device also includes a memory array sequencer for controlling the timing of external accesses and refresh accesses within the memory device. In general, the memory array sequencer ensures that the required refresh accesses are performed without interfering with any external accesses.

[0016] More specifically, the memory array sequencer synchronizes external accesses and refresh accesses with different edges of an external clock signal. In one embodiment, external accesses are synchronized (initiated) in response to rising edges of the external clock signal. The external accesses are then completed during the first half cycle of the clock period (e.g., while the clock signal has a high state). In this embodiment, pending refresh accesses are synchronized (initiated) in response to falling edges of the external clock signal. The refresh accesses are then completed during the second half cycle of the clock period (e.g., while the clock signal has a low state).

[0017] In another embodiment, external accesses are synchronized (initiated) in response to rising edges of the external clock signal. The external accesses are performed as quickly as possible. Pending refresh accesses are then synchronized (initiated) in response to the end of the external accesses. The refresh accesses are completed prior to the next rising edge of the external clock signal. This embodiment allows the external accesses and refresh accesses to be performed as quickly as possible, without being dependent on the duty cycle of the external clock signal.

[0018] The present invention will be more fully understood in view of the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a block diagram of a memory system in accordance with one embodiment of the present invention.

[0020]FIG. 2 is a block diagram of a refresh controller used in the memory system of FIG. 1 in accordance with one embodiment of the present invention.

[0021]FIG. 3 is a circuit diagram of a memory array sequencer used in the memory system of FIG. 1 in accordance with one embodiment of the present invention.

[0022]FIG. 4 is a waveform diagram illustrating the timing of various signals in the memory system of FIG. 1 in accordance with one embodiment of the present invention.

[0023]FIG. 5 is a circuit diagram of another memory array sequencer, which can be used to replace the memory array sequencer of FIG. 3 in another embodiment of the present invention.

[0024]FIG. 6 is a waveform diagram illustrating the timing of various signals in the memory system of FIG. 1, when the memory array sequencer of FIG. 5 is used in place of the memory array sequencer of FIG. 3.

DETAILED DESCRIPTION

[0025] In accordance with the present invention, an SRAM compatible device is designed using DRAM cells. This SRAM compatible device is hereinafter referred to as a one-transistor SRAM (1-T SRAM).

[0026]FIG. 1 is a block diagram of a 1-T SRAM system 100 in accordance with one embodiment of the present invention. 1-T SRAM system 100 includes DRAM array 101, 2-to-1 multiplexer 102, AND gate 103, input signal registers 120, refresh controller 121 and memory array sequencer 122. DRAM array 101 includes 64 DRAM memory banks 0-63, read data amplifier 70 and write data buffer 80. Each of DRAM banks 0-63 includes 32 rows and 512 columns of DRAM memory cells, as well as the associated address decoders, word line drivers, sense-amplifiers and column multiplexers, which are not shown, but are understood by those of ordinary skill in the art. The column multiplexers of DRAM memory banks 0-63 are connected in parallel to read data amplifiers 70 and write data buffer 80. Each of DRAM banks 0-63 is capable of storing 512 32-bit words. The total capacity of DRAM memory banks 0-63 is therefore 32K 32-bit words.

[0027] The external interface of 1-T SRAM system includes a 32-bit input data bus for providing an input data word DIN[31:0] to write data buffer 80, a 32-bit output data bus for providing an output data word DOUT[31:0] from read data amplifier 70, a write/read indicator line for receiving write/read indicator signal WR#, a clock input terminal for receiving an external clock signal CLK, a 14-bit address bus for receiving external address bits EA[14:0], an address strobe line for receiving an address strobe signal ADS#, and a reset line for receiving a reset signal RST#. As used herein, the # symbol indicates that a signal is active low. Note that the signals listed above do not include any signals specifically relating to refresh accesses of DRAM banks 0-63.

[0028] All of the external signals are referenced to the rising edge of the CLK signal. The external address signal EA[14:0] is divided into 3 fields: a 4-bit word (column) address EA[14:11] (which identifies one of the sixteen 32-bit words in a row), a 6-bit bank address EA[10:5] (which identifies one of the 64 memory banks 0-63), and a 5-bit row address A[4:0] (which identifies one of the 32 rows in a memory bank).

[0029] An external device initiates an access to 1-T SRAM system 100 by asserting a logic low address strobe signal ADS#, driving the write/read indicator signal WR# to the desired state (high for write and low for read), and providing an external address EA[14:0]. The ADS#, WR# and EA[14:0] signals are all registered (i.e., latched) into input signal registers 120 at the rising edge of CLK signal. Input signal registers 120 provide the latched ADS#, WR# and EA[14:0] signals as latched output signals LADS#, LWR# and LEA[14:0], respectively. The latched external address LEA[14:0] is divided into two groups of address signals. Address signal LEA[14:11] contains the column address of the memory cells selected for the external access, and address signal LEA[10:0] contains the row and bank address of the memory cells selected for the external access. Address signal LEA[14:11] is provided to the column decoder circuitry in each of DRAM banks 0-63. Address signal LEA[10:0] is provided to an input bus of 2-to-1 multiplexer 102.

[0030] The latched address strobe signal LADS# is provided to memory array sequencer 122 and to an inverting input terminal of AND gate 103. The latched write/read indicator signal LWR# is provided to read data amplifier 70 and write data buffer 80.

[0031] Refresh accesses to DRAM array 101 are managed by refresh controller 121. Refresh controller 121 initiates a refresh access by activating a refresh request signal RREQ high in response to the external clock signal CLK. As described in more detail below, refresh controller 121 activates the refresh request signal RREQ high once every 64 clock periods. Refresh controller 121 also provides an 11-bit refresh address RFA[10:0], which identifies the bank and row to be refreshed in DRAM array 101. The refresh address RFA[10:0] is incremented each time the refresh request signal is activated.

[0032]FIG. 2 is a block diagram of refresh controller 121 in accordance with one embodiment of the present invention. Refresh controller 121 includes a modulo-64 counter 201 for generating the refresh request signal RREQ, and an 11-bit binary counter 202 for generating the bank and row address RFA[10:0] for the current refresh operation. Counters 201 and 202 are reset to zero counts when the reset signal RST# is activated low during the initialization of memory system 100. After the RST# signal is de-activated high, modulo-64 counter 201 begins counting from 0 to 63, with counter 201 being incremented by one in response to each rising edge of the external clock signal CLK. When modulo-64 counter 201 reaches a full count of 63, this counter 201 drives the refresh request signal RREQ high for one period of the CLK signal. Modulo-64 counter 201 then wraps around to a zero count in response to the next rising edge of the CLK signal.

[0033] The falling edge of the refresh request signal RREQ increments the count of 11-bit binary counter 202 by one. The output of 11-bit binary counter 202, provides the refresh address RFA[10:0], which identifies the bank and row to be refreshed in DRAM array 101. After 11-bit binary counter 202 reaches a full count of “111 1111 1111”, the count will automatically wrap around to a zero count in response to the next rising edge of the refresh request signal RREQ. In this manner, refresh controller 121 provides a refresh address RFA[10:0] that traverses the entire row and bank address space of DRAM array 101.

[0034] Memory array sequencer 122 generates the conventional DRAM control signals RAS# (row access), SEN# (sense amplifier enable), CAS# (column access), and PRC# (precharge) for controlling the operations of memory array 101. The functionality of the RAS#, SEN#, CAS# and PRC# control signals in accessing a DRAM array is understood by those of ordinary skill in the art. For example, the functionality of these control signals is described in detail in U.S. Pat. No. 6,078,547, “Method and Structure for Controlling Operation of a DRAM Array”, by Wingyu Leung, which is hereby incorporated by reference.

[0035]FIG. 3 is a circuit diagram of memory array sequencer 122 in accordance with one embodiment of the present invention. Memory array sequencer 122 includes delay lines 301-304, OR gates 311-318, one-shot circuits 331-332, AND gate 341, and flip-flops 351-352. Flip-flop 351 includes cross-MS-coupled NAND gates 321-322, and flip-flop 352 includes cross-coupled NAND gates 323-324. Delay lines 301, 302, 303 and 304 introduce signal delays d1, d2, d3, and d4, respectively. In general, memory array sequencer 122 activates the RAS#, SEN#, CAS# and PRC# signals at predetermined times during half of a CLK period. As a result, an external access (read or write) may be performed during one half of a CLK period, and a refresh access may be performed during the other half of the same CLK period. In the described embodiment, external accesses are performed during the half clock period that the CLK signal has a logic high state, and refresh accesses are performed during the half clock period that the CLK signal has a logic low state.

[0036] When 1-T SRAM system 100 is started, the RST# signal is activated low, thereby setting flip-flops 351 and 352, such that these flip-flops provide logic high output signals to AND gate 341. The RST# signal is then de-activated high, thereby allowing flip-flops 351 and 352 to operate in response to the other applied signals. When there is no external access to system 100, the latched address strobe signal LADS# is deactivated high. When there is not refresh access pending in system 100, the refresh request signal RREQ is de-activated low. Under these conditions, OR gates 311 and 312 provide logic high signals to one-shot circuits 331 and 332, respectively. In response, one-shot circuits 331-332 provide logic high signals to flip-flops 351-352, respectively, thereby causing flip-flops 351-352 to continue to provide logic high output signals to AND gate 341. The logic high output of AND gate 341 de-activates the RAS#, SEN#, CAS# and PRC# control signals high. At this time, memory array 101 is idle.

[0037] As described in more detail below, the latched address strobe signal LADS# is activated low during an external access. The low state of the LADS# signal, along with the high state of the CLK signal causes the output of OR gate 311 to go low. The falling edge at the output of OR gate 311 causes one-shot circuit 331 to generate a negative going pulse having a width substantially shorter than half of the CLK period. This negative pulse resets the output of flip-flop 351 to a logic low value. The logic low value provided by flip-flop 351 causes AND gate 341 to activate the RAS# signal low. The low state of the RAS# signal propagates through delay lines 301, 302, 303 and 304 (as well as OR gates 315-317) to drive signals SEN#, CAS# and PRC# low in sequence. More specifically, the low RAS# signal propagates through delay line 301 and OR gate 315 to activate the SEN# signal low after a delay of d1. The low SEN# signal then propagates through delay line 302 and OR gates 316-317 to activate the CAS# signal low after an additional delay of d2. Note that the logic high CLK signal applied to the inverting input terminal of OR gate 317 allows the logic low signal from OR gate 316 to be passed as the low CAS# signal. The low CAS# signal then propagates through delay line 303 to activate the PRC# signal low after an additional delay of d3.

[0038] The logic low signal provided by delay line 303 propagates through delay line 304 after an additional delay of d4. The logic low output of delay line 304 is provided to the inverting input terminal of OR gate 318. As a result, the PRC# signal is de-activated high a delay d4 after being activated low.

[0039] The low state of the output of delay line 303 and the high state of the CLK signal cause OR gate 313 to provide a logic low value to flip-flop 351, thereby setting the output of flip-flop 351 to a logic high state. In response, AND gate 341 deactivates the RAS# signal to a logic high state. The logic high RAS# signal is applied to OR gates 315 and 316, thereby causing the SEN# and CAS# signals, respectively, to be immediately de-activated high. The logic high value provided by OR gate 316 propagates through delay line 303 after a delay of d3, thereby causing OR gate 318 to continue to de-activate the PRC# signal high. The logic high value provided by delay line 303 propagates through delay line 304 after a delay of d4, and is applied to the inverting input terminal of OR gate 318. In this manner, the output signals provided by delay lines 301-304 are reset to their original logic high states, such that these delay lines are ready for the next access.

[0040] The total delay introduced by delay lines 301, 302, 303, and 304 is designed to be less than or equal to a half period of the CLK signal. Notice that the RAS# signal is recovered to a logic high state before the CLK signal transitions from the high state to a low state. As a result, another memory cycle can be started at the falling edge of the CLK signal. To ensure that the SEN# and CAS# signals are generated properly during back-to-back memory cycles, the outputs of delay lines 301 and 302 are required to be deactivated high before the RAS# signal is activated low again.

[0041] In the present embodiment, the delay d4 introduced by delay line 304 is longer than the delay dl introduced by delay line 301 or the delay d2 introduced by delay line 302. As a result, when the PRC# signal is de-activated high, delay lines 301 and 302 are already providing logic high output signals. In other embodiments, delay line 301 or delay line 302 can have a longer delay than delay line 304. However, in such embodiments, a fast reset must be provided on delay lines 301 and 302, thereby ensuring that the output signals provided by delay lines 301 and 302 recover to logic high states before the activation period of the PRC# signal expires.

[0042] A memory refresh operation is performed when the refresh request signal RREQ is activated to a logic high value and the CLK signal has a logic low value. That is, refresh operations are only performed during the half period that the CLK signal has a logic low state. The high state of the RREQ signal and the low state of CLK signal causes OR gate 312 to provide a logic low output signal to one-shot circuit 332. In response, one-shot circuit 332 provides a negative going pulse having a pulse width substantially shorter than a half period of the CLK signal. The negative pulse resets the output of flip-flop 352 to a logic low state. This logic low output signal provided by flip-flop 352 is applied to an input terminal of AND gate 341. In response, AND gate 341 activates the RAS# signal low. The low state of RAS# signal propagates through delay line 301, thereby causing the SEN# signal to be activated low after delay dl. The low state of the SEN# signal propagates through delay line 302 after delay d2, thereby causing OR gate 316 to provide a logic low output signal to OR gate 317. Because the inverting input terminal of OR gate 317 receives a logic low CLK signal at this time, the CAS# signal remains de-activated high. The CAS# signal is thereby suppressed during the refresh access (because the refresh access does not involve a column access).

[0043] The logic low signal provided by OR gate 316 propagates through delay line 303, thereby providing a low signal to the non-inverting input terminal of OR gate, and causing the PRC# signal to be activated low after delay d3. The logic low state of the output signal provided by delay line d3 propagates through delay line 304, thus providing a logic low signal to the inverting input terminal of OR gate 318 after delay d4. OR gate 318 de-activates the PRC# signal high in response to the logic low signal provided to the inverting input terminal of OR gate. The low state of the output signal provided by delay line 303 and the low state of the CLK signal cause OR gate 314 to provide a logic low output signal to flip-flop 352. In response, flip-flop 352 provides a logic high signal to AND gate 341, thereby causing AND gate 341 to deactivate the RAS# signal high. The high state of the RAS# signal causes OR gate 315 to provide a logic high output signal, thereby causing the SEN# signal to be deactivated high. The high state of the RAS# signal also causes OR gate 316 to provide a logic high output signal. After a delay of d3, the logic high output signal provided by OR gate 316 propagates through delay line 303, to the non-inverting input terminal of OR gate 318, thereby causing OR gate 318 to continue to de-activate the PRC# signal high. The logic high value provided by delay line 303 propagates through delay line 304 after a delay of d4, and is applied to the inverting input terminal of OR gate 318. In this manner, the output signals provided by delay lines 301-304 are reset to their original logic high states, such that these delay lines are ready for the next access.

[0044] Returning now to FIG. 1, multiplexer 102 routes either the latched external address LEA[10:0] or the refresh address RFA[10:0] to memory array 101 as the row/bank address RBA[10:0]. Multiplexer 102 is controlled by the output signal provided by AND gate 103. During an external access, the LADS# signal is activated low and the CLK signal is high, thereby causing AND gate 103 to provide a logic high signal to the control terminal of multiplexer 102. In response, multiplexer 102 routes the latched external address LEA[10:0] to array 101. If an external access is not being performed, AND gate 103 provides a logic low signal to the control terminal of multiplexer 102, thereby causing the refresh address RFA[10:0] to be routed to array 101.

[0045]FIG. 4 is a waveform diagram that illustrates the timing of various signals in 1-T SRAM system 100 in accordance with one embodiment of the present invention. In the described example, a read access is performed during the first half of clock cycle T1. A refresh operation is performed during the second half of clock cycle T1. A write access is performed during the first half of clock cycle T2. Memory system 100 is idle during the second half of clock cycle T2, as there is no pending refresh at this time. To simplify the timing in the description below, all of the logic gates are assumed to have a negligible delay compared to the period the CLK signal and the delays d1-d4 introduced by delay lines 301-304.

[0046] Read Access

[0047] The external accessing client provides a low ADS# signal, a low WR# signal and an external address signal EA[14:0] before the rising edge of clock cycle T1. The low WR# signal specifies a read operation, and the external address signal EA[14:0] specifies the read address within DRAM array 101. At the rising clock-edge of cycle T1, the ADS#, WR# and EA[14:0] signals are latched into input signal registers 120, and provided as the LADS#, LWR# and LEA[14:0] signals, respectively.

[0048] Within refresh controller 121, the rising clock-edge of cycle T1 increments modulo-64 counter 201 to a full count thereby causing the refresh request signal RREQ to be activated high. The high state of the RREQ signal increments 11-bit binary counter by one count, such that the refresh address RFA[10:0] has a value represented by “FA”. The refresh address FA identifies the bank and row address for the pending refresh operation. The refresh address FA is provided to multiplexer 102, as the refresh address signal RFA[10:0].

[0049] The logic low LADS# signal and the logic high CLK signal cause AND gate 103 (FIG. 1) to provide a logic high control signal to multiplexer 102. In response, multiplexer 102 routes the bank/row information of the latched address LEA[10:0] to memory array 101 as the RBA[10:0] signal. The column information of the latched address LEA[14:11] is also provided to memory array 101. In response, decoders (not shown) identify an addressed bank, an addressed row and an addressed set of columns of the present read access.

[0050] The logic low LADS# signal and the logic high RREQ are provided to memory array sequencer 122 (FIG. 3). The low state of the LADS# signal and the high state of the CLK signal causes the RAS# signal to be activated low in the manner described above. The low state of the RAS# signal propagates through delay line 301, thereby causing the SEN# signal to be activated low after a delay of d1. The low state of the SEN# signal propagates through delay line 302, thereby causing the CAS# signal to be activated low after a delay of d2. Finally, the low state of the CAS# signal propagates through delay line 303, thereby causing the PRC# signal to be activated low after a delay of d3. In this manner, the RAS#, SEN#, CAS# and PRC# signals are sequentially activated low.

[0051] The logic low RAS# signal causes the data in the addressed row of the addressed bank (as specified by LEA[10:0]) to be driven to the sense amplifiers of the addressed bank. The logic low SEN# signal causes this row of data to be latched in the sense amplifiers of the addressed bank. The logic low CAS# signal causes the sense amplifiers corresponding with the addressed set of columns (as specified by LEA[14:11]) to be coupled to read data amplifier 70. The logic low PRC# signal causes the data word (RDA) read from DRAM array 101 to be latched into read data amplifier 70 and provided as the data output signal DOUT[31:0].

[0052] Within memory array sequencer 122, the low state of the PRC# signal also causes the RAS#, SEN# and CAS# signals to be deactivated high in the manner described above. Within memory array 101, the low state of the PRC# signal turns off the word line of the addressed row in the addressed bank, turns off the sense amplifiers in the addressed bank, and precharges the bit lines of the addressed bank, thereby preparing memory array 101 for the next operation. After a delay of d4, the logic low output signal provided by delay line 303 propagates through delay line 304, thereby deactivating the PRC# signal high in the manner described above, and completing the read access.

[0053] Refresh Access

[0054] In the second half of clock cycle T1, the low state of the CLK signal causes AND gate 103 to provide a logic low signal to the control terminal of multiplexer 102. In response, multiplexer 102 routes the refresh address RFA[10:0] to memory array 101 as the RBA[10:0] signal. In response, decoders (not shown) identify an addressed bank and an addressed row of the present refresh access.

[0055] Within memory array sequencer 122, the low state of the CLK signal and the high state of the RREQ signal cause the RAS# signal to be activated low in the manner described above. The low state of the RAS# signal propagates through delay line 301, thereby causing the SEN# signal to be activated low after a delay of d1. The low state of the SEN# signal propagates through delay line 302, with a delay of d2, and then through delay line 303, with a delay of d3, thereby causing the PRC# signal to be activated low after a delay of d2 plus d3. Note that the logic low CLK signal prevents OR gate 317 from activating a logic low CAS# signal, as column access is not required during a refresh operation. In this manner, the RAS#, SEN# and PRC# signals are sequentially activated low.

[0056] The logic low RAS# signal causes the data in the addressed row of the addressed bank (as specified by RFA[10:0]) to be driven to the sense amplifiers of the addressed bank. The logic low SEN# signal causes this row of data to be latched in the sense amplifiers of the addressed bank. The sense amplifiers resolve the data values to a full signal swing, thereby refreshing the data from the addressed row.

[0057] Within memory array sequencer 122, the low state of the PRC# signal causes the RAS# and SEN# signals to be deactivated high in the manner described above. Within memory array 101, the low state of the PRC# signal turns off the word line of the addressed row in the addressed bank, turns off the sense amplifiers in the addressed bank, and precharges the bit lines of the addressed bank, thereby preparing DRAM array 101 for the next operation. After a delay of d4, the PRC# signal is deactivated high, thereby completing the refresh access.

[0058] Write Access

[0059] The external accessing client provides a low ADS# signal, a high WR# signal, a write data value DIN[31:0] and an external address signal EA[14:0] before the rising edge of clock cycle T2. The high WR# signal specifies a write operation, and the external address signal EA[14:0] specifies the write address within DRAM array 101. At the rising clock-edge of cycle T2, the ADS#, WR# and EA[14:0] signals are latched into input signal registers 120, and provided as the LADS#, LWR# and LEA[14:0] signals, respectively. In addition, the write data value DIN[31:0] is latched into write data buffer 80.

[0060] The logic low LADS# signal and the logic high CLK signal cause AND gate 103 (FIG. 1) to provide a logic high control signal to multiplexer 102. In response, multiplexer 102 routes the bank/row information of the latched address LEA[10:0] to DRAM array 101 as the RBA[10:0] signal. The column information of the latched address LEA[14:11] is also provided to memory array 101. In response, decoders (not shown) identify an addressed bank, an addressed row and an addressed set of columns of the present write access.

[0061] Within refresh controller 121, the modulo-64 counter 201 is incremented to a zero count in response to the rising edge of the CLK signal, thereby causing the refresh request signal RREQ to be deactivated low.

[0062] The logic low LADS# signal and the logic low RREQ signal are provided to memory array sequencer 122 (FIG. 3). The low state of the LADS# signal and the high state of the CLK signal cause the RAS#, SEN#, CAS# and PRC# signals to be sequentially activated low in the manner described above.

[0063] The logic low RAS# signal causes the data in the addressed row of the addressed bank (as specified by LEA[10:0]) to be driven to the sense amplifiers of the addressed bank. The logic low SEN# signal causes this row of data to be latched in the sense amplifiers of the addressed bank. The logic low CAS# signal causes the sense amplifiers corresponding with the addressed set of columns (as specified by LEA[14:11]) to be coupled to write data buffer 80, thereby overwriting the data in the sense amplifiers corresponding with the addressed set of columns with the write data value DIN[31:0]. These sense amplifiers, in turn, couple the write data value to the corresponding memory cells in the addressed row.

[0064] Within memory array sequencer 122, the low state of the PRC# signal causes the RAS#, SEN# and CAS# signals to be deactivated high in the manner described above. Within DRAM array 101, the low state of the PRC# signal turns off the word line of the addressed row in the addressed bank, turns off the sense amplifiers in the addressed bank, and precharges the bit lines of the addressed bank, thereby preparing memory array 101 for the next operation. After a delay of d4, the PRC# signal is deactivated high, thereby completing the write access.

[0065] In the foregoing manner, 1-T SRAM system 100 implements refresh accesses without interfering with external accesses, and without requiring an external refresh indicator signal. In the embodiment described above, the RAS#, SEN#, CAS# and PRC# signals are only activated when an external access or refresh operation is being performed. This results in power savings in 1-T SRAM system 100.

[0066] Alternate Embodiment

[0067] In the embodiment described above, the external accesses are performed during one half of the clock period (i.e., when the CLK signal is high), and the refresh operations are performed during the other half of the clock period (i.e., when the CLK signal is low). Operation of 1T SRAM system 100 therefore depends on both the high period and the low period of the CLK signal. Consequently, the operation and performance of 1-T SRAM system 100 is affected by the duty-cycle of the external clock signal CLK. Because a refresh access does not involve a column access operation, the memory cycle time for a refresh access is shorter than the memory cycle time for an external access. Consequently, 1-T SRAM system 100 would be capable of operating at higher clock frequencies if the memory cycle time of refresh accesses is optimized. In general, the shortest possible clock period (i.e., the highest possible clock frequency) exists when the clock period is equal to memory cycle time of an external access plus the memory cycle time of a refresh access. It is therefore desirable to have a memory system that operates independent of the clock duty-cycle, such that the memory system can operate in response to the shortest possible clock period.

[0068]FIG. 5 is a circuit diagram of memory array sequencer 500, which is used in another embodiment of the present invention. In this embodiment, memory array sequencer 500 replaces memory array sequencer 122 (FIGS. 1, 3). The construct of the other functional blocks of memory system 100 remains the same. As described in more detail below, memory array sequencer 500 enables memory system 100 to operate independent of the duty-cycle of the CLK signal.

[0069] Memory array sequencer 500 includes delay lines 501-505, OR gates 511-520, AND gates 521-523, one-shot circuits 531-533, and flip-flops 551-554, which include cross-coupled NAND gate pairs 541-542, 543-544, 545-546 and 547-548, respectively. In general, delay lines 501-503 introduce the same signal delays (d1, d2, d3) as delay lines 301-303, respectively. Delay lines 504 and 505 introduce the same delays (d1, d3) as delay lines 501 and 503, respectively. Delay lines 501-503 serve two functions. First, during an external (read/write) access, delay lines 501-503 generate the timing control for the memory access operations. That is, delay lines 501-503 control the timing of the SEN#, CAS# and PRC# signals during an external access cycle. Second, delay lines 501-503 control the timing of the beginning of a refresh period. Delay lines 504 and 505 generate the timing control for the refresh access operations. That is, delay lines 504-505 control the timing of the SEN# and PRC# signals during a memory refresh cycle. External access timing control signals aRAS#, aSEN#, aPRC# and their counterpart refresh timing control signals rRAS#, rSEN# and rPRC# are logically AND'ed to form the array control timing signals RAS#, SEN# and PRC#, respectively. The CAS# signal, which controls the column operation of the array, is activated only during external access cycles.

[0070] Initial Generation of aRAS#, aSEN#, CAS#, aPRC#

[0071]FIG. 6 is a waveform diagram illustrating the operation of 1-T SRAM system 100 when using memory array sequencer 500. During the first clock cycle T1, there are no external accesses or refresh accesses pending in the memory system. During the second clock cycle T2, both a read access and a refresh access are pending. Although a write access is not specifically described in FIG. 6, it is understood that memory array sequencer 500 generates the same signals during read and write accesses.

[0072] The first clock cycle T1 is representative of the state of memory array sequencer 500 after memory system 100 has been initialized (although it is understood that a refresh request would not be generated the cycle after initialization). When 1-T SRAM system 100 is started, the RST# signal is activated low, thereby setting flip-flops 551-554, such that these flip-flops provide logic high output signals. During clock cycle T1, there is no external access or refresh access to system 100, so the latched address strobe signal LADS# is de-activated high and the refresh request signal RREQ is de-activated low. As a result, OR gates 511 and 512 provide logic high signals to one-shot circuits 532 and 533, respectively. In response, one-shot circuits 532-533 provide logic high signals to flip-flops 552-553, respectively, thereby causing flip-flops 552-553 to continue to provide logic high output signals. The output signal of flip-flop 552 is referred to as the aRAS# signal. As described below, the aRAS# signal is used to activate the RAS# signal during an external access. The logic high aRAS# signal is provided to AND gate 521.

[0073] The logic high LADS# signal causes OR gates 518-520 to provide logic high output signals aSEN#, CAS# and aPRC#, respectively. The aSEN# and aPRC# signals are used to activate the SEN# and PRC# signals during an external access. The CAS# signal always corresponds with an external access (because the CAS# signal is not required during a refresh access). The logic high aSEN# and aPRC# signals are provided to AND gates 522 and 523, respectively.

[0074] Initial Generation of rRAS#, rSEN#, rPRC#

[0075] The logic high output signal of flip-flops 553 and 554 cause OR gate 513 to provide a logic high signal to OR gate 516. In response, OR gate 516 provides a logic high output signal. The output signal of OR gate 516 is referred to as the rRAS# signal. As described below, the rRAS# signal is used to activate the RAS# signal during a refresh access. The logic high rRAS# signal is provided to AND gate 521. Because both the aRAS# and rRAS# signals initially have logic high values, AND gate 521 initially provides a logic high (de-activated) RAS# signal.

[0076] The logic high rRAS# signal is routed through delay line 504 to OR gate 517, thereby causing OR gate 517 to provide a logic high rSEN# signal. The rSEN# signal is used to activate the SEN# signal during a refresh access. The logic high rSEN# signal is provided to AND gate 522. Because both the aSEN# and rSEN# signals initially have logic high values, AND gate 522 initially provides a logic high (deactivated) SEN# signal.

[0077] The logic high rSEN# signal is routed through delay line 505 to provide a logic high rPRC# signal. The rPRC# signal is used to activate the PRC# signal during a refresh access. The logic high rPRC# signal is provided to AND gate 523. Because both the aPRC# and rPRC# signals initially have logic high values, AND gate 523 initially provides a logic high (de-activated) PRC# signal.

[0078] Generation of aS#, C# and aP#

[0079] Initially, flip-flop 551 provides a logic high output signal to delay line 501. This logic high output signal propagates through delay line 501 to OR gate 514. In response, OR gate 514 provides a logic high aS# signal. As described below, the aS# signal is used to activate the aSEN# signal.

[0080] The logic high aS# signal propagates through delay line 501 to OR gate 515. In response, OR gate 515 provides a logic high C# signal. As described below, the C# signal is used to activate the CAS# signal.

[0081] The logic high C# signal propagates through delay line 503, thereby providing a logic high aP# signal. As described below, the aP# signal is used to activate the aPRC# signal, and to signal the start of a refresh access.

[0082] At the rising edge of each clock cycle, including clock cycle T1, one-shot circuit 531 is activated, such that one-shot circuit 531 generates a negative going pulse having a duration substantially shorter than one half the CLK period. This negative pulse resets the output of flip-flop 551 to a logic low value. The logic low value provided by flip-flop 551 propagates through delay line 501, with delay d1, to OR gate 514. In response, OR gate 514 provides a logic low aS# signal.

[0083] The logic low aS# signal propagates through delay line 502, with delay d2, to OR gate 515. In response, OR gate 515 provides a logic low C# signal.

[0084] The logic low C# signal propagates through delay line 503, with delay d3, thereby providing a logic low aP# signal. The logic low aP# signal causes the aS# and C# signals to transition to logic high states (through OR gates 514 and 515, respectively). The logic low aP# signal also returns (sets) the output of flip-flop 551 to a logic high state. The logic high C# signal propagates through delay line 503, with delay d3, thereby causing the aP# signal to transition back to a logic high state. Note that the aS#, C# and aP# signals are generated in this sequence during every cycle of the CLK signal, regardless of the states of the LADS# or RREQ signals. That is, the aS#, C# and aP# signals are generated in this sequence whether or not there is an external access and/or refresh access pending in memory system 100.

[0085] External Access

[0086] The external accessing client provides a low ADS# signal, a low WR# signal and an external address signal EA[14:0] before the rising edge of clock cycle T2. The low WR# signal specifies a read operation, and the external address signal EA[14:0] specifies the read address within array 101. At the rising clock-edge of cycle T2, the ADS#, WR# and EA[14:0] signals are latched into input signal registers 120, and provided as the LADS#, LWR# and LEA[14:0] signals, respectively. The functionality of the WR#, LWR#, EA[14:0] and LEA[14:0] signals has been described above in connection with FIGS. 1-4. Because these signals are not relevant to the operation of memory array sequencer 500, these signals are not discussed further in the present example.

[0087] Within refresh controller 121, the rising clock-edge of cycle T2 increments modulo-64 counter 201 to a full count thereby causing the refresh request signal RREQ to be activated high. The high state of the RREQ signal increments 11-bit binary counter by one count, such that the refresh address RFA[10:0] has a value represented by “FA”. Because the functionality of the refresh address RFA[10:0] has been described above, this description is not repeated in the present example.

[0088] As described above, the LADS# signal is activated low at the rising edge of clock cycle T2. The low state of the LADS# signal, along with the high state of the CLK signal causes the output of OR gate 511 to go low. The falling edge at the output of OR gate 511 causes one-shot circuit 532 to generate a negative going pulse having a width substantially shorter than half of the CLK period. This negative pulse resets the output of flip-flop 552 (i.e., the aRAS# signal) to a logic low value. This logic low aRAS# signal causes AND gate 521 to activate the RAS# signal low.

[0089] The rising edge of clock cycle T2 also causes one-shot circuit 551 to generate a negative going pulse having a width substantially shorter than half of the CLK period. This negative pulse resets the output of flip-flop 551 to a logic low value. This logic low value propagates through delay lines 501, 502 and 503, thereby sequentially activating and deactivating the aS#, C# and aP# signals in the manner described above for clock cycle T1. Because the LADS# signal has a logic low value at this time, OR gates 518, 519 and 520 effectively pass the aS#, C# and aP# signals as the aSEN#, CAS# and aPRC# signals, respectively. In addition, AND gates 522 and 523 effectively pass the aSEN# and aPRC# signals as the SEN# and PRC# signals, respectively. When the aP# signal is activated low, flip-flop 552 is set, thereby de-activating the aRAS# signal high. In response, to the high aRAS# signal, AND gate 522 deactivates the RAS# signal high. The read access is performed in response to the sequentially activated and deactivated RAS#, SEN#, CAS# and PRC# signals. As described in more detail below, the aP# signal is used to coordinate the timing of the refresh access.

[0090] Refresh Access

[0091] Turning now to the refresh access, the rising edge of clock cycle T2 (along with the logic high RREQ signal) causes OR gate 512 to provide a logic low signal to one-shot circuit 533. In response, one-shot circuit 553 generates a negative going pulse having a width substantially shorter than half of the CLK period. This negative pulse resets the output of flip-flop 553 to a logic low value, which is provided to OR gate 513. However, because flip-flop 554 provides a logic high signal to OR gate 513, this OR gate continues to provide a logic high output signal. As a result, the rRAS# signal remains deactivated high.

[0092] The aP# signal is applied to an input terminal of flip-flop 554. When the aP# signal is activated low, flip-flop 554 is reset, thereby causing this flip-flop 554 to provide a logic low output signal to OR gate 513. However, because an inverting input terminal of OR gate 513 is coupled to receive the aP# signal, OR gate 513 continues to provide a logic high output signal at this time. As a result, the rRAS# signal remains deactivated high.

[0093] As described above, the aP# signal is deactivated high after a delay of d3. However, this transition of the aP# signal does not change the output signal of flip-flop 554. As a result, OR gate 513 provides a logic low output signal in response to the rising edge of the aP# signal. The logic low output signal of OR gate 513 is provided to OR gate 516. In response, OR gate 516 activates the rRAS# signal low. This logic low rRAS# signal is effectively routed through AND gate 521, thereby providing a logic low RAS# signal for the refresh access. In this manner, the RAS# signal of the refresh access is activated in response to the deactivated aP# signal. Stated another way, the refresh access is automatically initiated at the conclusion of the external (read) access. This advantageously allows the external access and the refresh access to be implemented in the shortest possible time.

[0094] The logic low rRAS# signal propagates through delay line 504 (with delay d2) to OR gate 517. In response, OR gate 517 activates the rSEN# signal low. This logic low rSEN# signal is effectively routed through AND gate 522, thereby providing a logic low SEN# signal for the refresh access.

[0095] The logic low rSEN# signal propagates through delay line 505 (with delay d3), thereby activating the rPRC# signal low. This logic low rPRC# signal is effectively routed through AND gate 523, thereby providing a logic low PRC# signal for the refresh access.

[0096] The logic low rPRC# signal causes the rRAS# and rSEN# signals to be deactivated high (through OR gates 516 and 517, respectively). The logic low rPRC# signal is also applied to input terminals of flip-flops 553 and 554, thereby setting the output signals of these flip-flops to logic high values, and preparing these flip-flops for the next refresh access. The logic high deactivated rSEN# propagates through delay line 505, thereby deactivating the rPRC# signal high after delay d2. At this time, memory array sequencer 500 has been returned to its initial state, and is ready for the next clock cycle.

[0097] Notice that if a refresh access were not pending in clock cycle T2, the RREQ signal would not be activated high. As a result, one-shot 533 would not be activated, and flip-flop 533 would continue to provide a logic high output signal to OR gate 513. Under these conditions, the rRAS#, rSEN# and rPRC# signals would not be activated low during this cycle, and the RAS#, SEN# and PRC# would not be activated for a second time during this cycle. That is, a refresh access will not be performed if a refresh access is not pending.

[0098] Although the invention has been described in connection with several embodiments, it is understood that this invention is not limited to the embodiments disclosed, but is capable of various modifications, which would be apparent to a person skilled in the art. For example, although the read/write accesses have been described as occurring during the first half of a clock cycle, and the refresh operations have been described as occurring during the second half of a clock cycle, it is understood that this order may be reversed in other embodiments. The manner of performing this reversal would be apparent to one of ordinary skill in the art. Furthermore, although the refresh operations have been described as no activating the CAS# signal, it is understood that the CAS# signal may be activated during refresh operations in other embodiments. Moreover, although the above-described memory array sequencers are initially reset by the RST# signal, it is understood that these memory array sequencers are capable of resetting themselves in the absence of the RST# signal. For example, the DRAM memory banks can have different sizes in different embodiments. Similarly, different numbers of DRAM banks can be used. Moreover, buses having different widths than those described can be used in other embodiments. In addition, different logic can be used to provide the same results. Thus, the invention is limited only by the following claims. 

1. A memory system comprising: an array of memory cells, wherein each of the memory cells must be periodically refreshed to retain a data value; a terminal for receiving an external clock signal that periodically transitions between a first state and a second state; a terminal for receiving an external access signal that is activated to indicate a requested read or write access to the array; a refresh controller configured to periodically activate a refresh request signal; and a memory array sequencer coupled to receive the external clock signal, the external access signal, and the refresh request signal, the memory array sequencer being configured to implement a read or write access to the array when the external access signal is activated and the external clock signal transitions to the first state, the memory array sequencer further being configured to implement a refresh access to the array when the refresh request signal is activated and the external clock signal transitions to the second state.
 2. The memory system of claim 1 , wherein the memory array sequencer comprises: a first sequential logic circuit configured to activate a row access signal when the external access signal is activated and the external clock signal transitions to the first state; a first delay line configured to propagate the row access signal with a first delay to activate a sense amplifier enable signal; a second delay line configured to propagate the sense amplifier enable signal with a second delay to activate a column access enable signal; and a third delay line configured to propagate the column access enable signal with a third delay to activate a precharge signal.
 3. The memory system of claim 2 , wherein the first sequential logic circuit is further configured to de-activate the row access signal when the precharge signal is activated.
 4. The memory system of claim 3 , wherein the memory array sequencer further comprises a combinational logic circuit configured to de-activate the sense amplifier enable signal and the column access signal in response to the deactivated row access signal.
 5. The memory system of claim 2 , wherein the memory array sequencer further comprises a second sequential logic circuit configured to activate the row access signal when the refresh request signal is activated and the external clock signal transitions to the second state.
 6. The memory system of claim 5 , wherein the memory array sequencer further comprises a combinational logic circuit configured to de-activate the column access signal when the external clock signal is in the second state.
 7. The memory system of claim 1 , wherein the array is divided into a plurality of independent banks, wherein each of the banks includes 32 rows of memory cells or less.
 8. The memory system of claim 1 , wherein the refresh controller comprises a first counter that is incremented in response to the external clock signal, wherein the first counter activates the refresh request signal upon reaching a predetermined count.
 9. The memory system of claim 8 , wherein the refresh controller further comprises a second counter for providing a refresh address for selecting a row of the array for a refresh access.
 10. The memory system of claim 1 , wherein the memory array sequencer comprises: means for sequentially activating and deactivating a first set of access signals when the external access signal is activated and the external clock signal transitions to the first state; and means for sequentially activating and deactivating a second set of access signals when the refresh request signal is activated and the external clock signal transitions to the second state.
 11. The memory system of claim 10 , wherein first set of access signals includes a row access signal (PAS#), a sense amplifier enable signal (SEN#), a column access signal (CAS#) and a precharge signal (PRC#).
 12. The memory system of claim 11 , wherein the second set of access signals includes a row access signal (RAS#), a sense amplifier enable signal (SEN#) and a precharge signal (PRC#), but not a column access signal (CAS#).
 13. The memory system of claim 10 , wherein the second set of access signals includes a row access signal (RAS#), a sense amplifier enable signal (SEN#) and a precharge signal (PRC#), but not a column access signal (CAS#).
 14. The memory system of claim 10 , wherein the first set of access signals are the same as the second set of access signals.
 15. A method of operating a memory system having an array of memory cells requiring periodic refresh, the method comprising: receiving an external clock signal that periodically transitions between a first state and a second state; receiving an external access signal that is activated to indicate a requested read or write access to the array; periodically activating a refresh request signal; performing a read or write access to the array when the external access signal is activated and the external clock signal transitions to the first state; and performing a refresh access to the array when the refresh request signal is activated and the external clock signal transitions to the second state.
 16. The method of claim 15 , further comprising: activating a row access signal when the external access signal is activated and the external clock signal transitions to the first state; propagating the row access signal with a first delay to activate a sense amplifier enable signal; propagating the sense amplifier enable signal with a second delay to activate a column access enable signal; and propagating the column access enable signal with a third delay to activate a precharge signal.
 17. The method of claim 16 , further comprising propagating the precharge signal with a fourth delay to deactivate the precharge signal.
 18. The method of claim 15 , further comprising deactivating the row access signal when the precharge signal is activated.
 19. The method of claim 18 , further comprising deactivating the sense amplifier enable signal and the column access signal in response to the de-activated row access signal.
 20. The method of claim 19 , further comprising deactivating the precharge signal after the precharge signal has been activated for a duration of the fourth delay.
 21. The method of claim 16 , further comprising: activating the row access signal when the refresh request signal is activated and the external clock signal transitions to the second state; and de-activating the column access signal when the external clock signal is in the second state.
 22. A memory system comprising: an array of memory cells, wherein each of the memory cells must be periodically refreshed to retain a data value; a terminal for receiving an external clock signal; a terminal for receiving an external access signal that is activated to indicate a requested read or write access to the array; a refresh controller configured to periodically activate a refresh request signal; and a memory array sequencer coupled to receive the external clock signal, the external access signal, and the refresh request signal, the memory array sequencer including: means for sequentially activating and then deactivating a first set of timing signals; means for enabling the first set of timing signals to implement a read or write access to the array if the external access signal is activated; and means for sequentially activating and then deactivating a second set of timing signals to implement a refresh access to the array if the refresh request signal is activated, wherein one of the first and second sets of timing signals is activated in response to the de-activation of a timing signal in the other one of the first and second sets of timing signals.
 23. The memory system of claim 22 , further comprising means for activating the other one of the first and second sets of timing signals in response to the external clock signal.
 24. The memory system of claim 22 , wherein the refresh controller comprises a counter for providing a refresh address for selecting a row of the array for a refresh access.
 25. A method of operating a memory system having an array of memory cells requiring periodic refresh, the method comprising: receiving an external clock signal; receiving an external access signal that is activated to indicate a requested read or write access to the array; periodically activating a refresh request signal; sequentially activating and then de-activating a first set of timing signals; enabling the first set of timing signals to implement a read or write access to the array if the external access signal is activated; and sequentially activating and then de-activating a second set of timing signals to implement a refresh access to the array if the refresh request signal is activated, wherein one of the first and second sets of timing signals is activated in response to the deactivation of the other one of the first and second sets of timing signals.
 26. The method of claim 25 , further comprising activating the other one of the first and second sets of timing signals in response to the external clock signal.
 27. The method of claim 25 , further comprising incrementing a counter in the refresh controller in response to the activated refresh request signal, the counter providing a refresh address for selecting a row of the array for a refresh access.
 28. The method of claim 25 , wherein the read or write access and the refresh access are performed during a period of the external clock signal.
 29. The method of claim 25 , wherein the step of sequentially activating and then de-activating the first set of timing signals comprises: activating a first control signal when the external clock signal transitions to a first state; propagating the first control signal with a first delay to activate a second control signal; propagating the second control signal with a second delay to activate a third control signal; and propagating the third control signal with a third delay to activate a fourth control signal.
 30. The method of claim 29 , wherein the second control signal is a sense amplifier enable signal, the third control signal is a column access enable signal, and the fourth control signal is a precharge signal.
 31. The method of claim 30 , wherein the step of enabling the first set of timing signals comprises routing the sense amplifier enable signal, the column access enable signal, and the precharge signal to the array if the external access signal is activated.
 32. The method of claim 29 , wherein the step of sequentially activating and then de-activating the second set of timing signals comprises: de-activating the fourth control signal; activating a fifth control signal when the fourth control signal is de-activated; propagating the fifth control signal with a fourth delay to activate a sixth control signal; and propagating the sixth control signal with a fifth delay to activate a seventh control signal.
 33. The method of claim 32 , wherein the fifth control signal is a row access enable signal, the sixth control signal is a sense amplifier enable signal, and the seventh control signal is a precharge signal.
 34. The method of claim 25 , wherein the step of sequentially activating and then de-activating the first set of timing signals comprises: activating a first control signal when the external clock signal transitions to a first state and the refresh access signal is activated; propagating the first control signal with a first delay to activate a second control signal; propagating the second control signal with a second delay to activate a third control signal.
 35. The method of claim 34 , wherein the first control signal is a row access enable signal, the second control signal is a sense amplifier enable signal, and the third control signal is a precharge signal.
 36. The method of claim 34 , wherein the step of sequentially activating and then de-activating the second set of timing signals comprises: de-activating the third control signal; activating a fourth control signal when the third control signal is de-activated; propagating the fourth control signal with a third delay to activate a fifth control signal; propagating the fifth control signal with a fourth delay to activate a sixth control signal; and propagating the sixth control signal with a fifth delay to activate a seventh control signal.
 37. The method of claim 36 , wherein the fourth control signal is a row access enable signal, the fifth control signal is a sense amplifier enable signal, the sixth control signal is a column access enable signal, and the seventh control signal is a precharge signal.
 38. The method of claim 37 , wherein the step of enabling the second set of timing signals comprises routing the row access enable signal, the sense amplifier enable signal, the column access enable signal, and the precharge signal to the array if the external access signal is activated. 